The recovery of heat and chemicals from the waste liquor obtained when producing cellulose pulp conventionally by cooking a cellulose feed stock in an aqueous liquor includes dehydrating the waste liquor to a comparatively high content of solids, usually above 60%, and then spraying the liquor into a furnace, where it is burnt in a reducing atmosphere in the lower part of the furnace. Additional air is supplied to the upper part of the furnace in order to bring about a complete combustion. A substantial part of the recovered chemicals are reclaimed in a molten state at the bottom of the furnace, but about 10% will leave the furnace together with the combustion gases, and are reclaimed after cooling of the gases. The reclaiming usually takes place in an electrostatic precipitator, and the recovered chemicals are returned to the process.
Due to environmental restrictions the losses of chemicals from the process must be carefully watched, which will bring about an undesirable increase of the content of sulphur and chlorine in the chemicals taking part in the process. The increase of chlorine is especially noticeable if waste fluid from the bleachery is concentrated and burnt in the furnace, or if the base material, or the chemicals added include chlorides. An increased amount of chlorides in the smelt will lower the melting point thereof, which means that the protecting cover of solidified chemicals upon the tube walls of the furnace will be thinner. This will increase the risk of corrosive attacks upon said tube walls.
An increase of the sulphur content may be caused by collecting and burning malodorous sulphurous gases, or if sulphuric acid residues from the preparation of chlorine-dioxide and from the pine oil plant are used to cover chemical losses in the process cycle. An increased sulphur content, i.e. an increased amount of sulphur as compared to the base of the process liquor, which usually is sodium, will cause an increased partial pressure of the sulphurous gases in the lower part of the furnace, and thereby an increased risk for corrosion. It is obviously desirable to reduce the partial pressure of the sulphurous gases in the combustion gases, as well as to reduce the content of chlorine in the smelt, especially in the deposits upon the walls of the furnace.